With a relatively small number of such units it is not too inconvenient to provide them with individual connections enabling the exchange of messages between any two of them. In a more elaborate system it is known to use time-division multiplexing by assigning to each unit a respective time slot in a recurrent frame; such a system, however, does not operate efficiently when data are emitted by the several units at irregular intervals in the form of packets or bursts so that the average rate of utilization of each time slot is low. A further alternative, using a single bus for communication among all the processing units, requires rather complex logical circuitry for arbitrating, on the basis of different priority levels, among simultaneous requests for access to the bus by several such units.
An arrangement of the latter type, in which a common communication channel or bus is randomly accessible to the various processing units exchanging messages with a centralized station, is known as the Aloha System described by Norman Abramson and Franklin F. Kuo in Computer-Communication Networks published 1973 by Prentice-Hall, Inc. In that system the problem of possible interference between concurrently emitted messages is solved by blocking the transmission of any overlapping packets and thereafter attempting their retransmission at different times. As the likelihood of collisions increases with the number of processing units, the channel may remain unused for extended periods in a multi-unit system of this type.